The present invention relates to a thermal transfer image-receiving sheet and a process for producing the same. More particularly, the present invention is concerned with a thermal transfer image-receiving sheet capable of forming a high-quality image free from defects such as dropout and omission of dots on any transfer material.
Various thermal transfer processes are known in the art. One of them is a transfer process which comprises supporting a sublimable dye as a recording agent on a substrate sheet, such as paper or a polyester film, to form a thermal transfer sheet and forming various full color images on the transfer material, such as paper or a plastic film, provided with the dye-receiving layer. In this case, a thermal head of a printer is used as heating means, and a number of color dots of three or four colors are transferred to the transfer material, thereby reproducing a full color image of an original by the multicolor dots.
Since the color material used is a dye, the image thus formed is very sharp and highly transparent, so that the resultant image is excellent in the reproducibility and gradation and the quality of the image is the same as that of an image formed by the conventional offset printing and gravure printing. Further, according to this method, it is possible to form an image having a high quality comparable to a full color photographic image.
The above-described method, however, has a drawback that since the transfer material on which an image can be formed is limited to a plastic sheet dyeable with a dye, a paper provided with a dye-receiving layer previously formed thereon or other material, an image cannot be directly formed on general plain paper. It is a matter of course that an image can be formed even on general plain paper when a dye-receiving layer is previously formed on the surface of the paper. This, however, leads to an increase in the cost and is difficult to apply to general existing transfer materials, for example, post cards, paper for memorandum, letter paper, paper for a report, etc.
In order to solve the above-described problem, a proposal has been made on a receiving layer transfer sheet for simply forming a dye-receiving layer only in a necessary portion when an image is formed on a commercially available existing transfer material such as plain paper (see, for example, Japanese Patent Laid-Open Publication No. 264994/1987).
The use of the above-described receiving layer transfer sheet gives rise to no significant problem when the paper is a converted paper having a smooth surface. On the other hand, in the case of plain paper having a rough surface, a postcard and other paper, since a fiber is exposed on the surface and deteriorates the surface smoothness, the transfer of the receiving layer cannot be homogeneously conducted, which gives rise to occurrence of dropout or omission of a dot in the image formed on the receiving layer, so that a high-quality image cannot be formed.
The transfer of a receiving layer having an increased thickness is considered useful as a method for solving the above-described problems. In this case, however, the transferability and peelability of the receiving layer are poor. Further, since the receiving layer is relatively hard, no satisfactory cushioning property can be attained. These cause the thermal migration from a thermal head through a thermal transfer sheet to become heterogeneous, so that no high-quality image can be formed. Further, there is a problem that since the heat conveyed to the receiving layer is liable to diffuse, the energy efficiency is unfavorably low.
Accordingly, an object of the present invention is to solve the above-described problem and to provide a thermal transfer image-receiving sheet which enables a high-quality and high-density image free from dropout and omission to be formed with a good thermal energy efficiency even on a rough paper not having a smooth surface.